Recent cars are equipped with an electronic control apparatus. The electronic control apparatus includes a solenoid drive device for driving a solenoid. For example, an automatic transmission car (automatic car) is equipped with a solenoid for controlling the oil pressure for gear change, and a solenoid drive device for driving the solenoid by flowing an electric current therethrough. For example, Unexamined Japanese Patent Application KOKAI Publication No. H3-177668, Unexamined Japanese Patent Application KOKAI Publication No. H4-50550, or Unexamined Japanese Patent application KOKAI Publication No. H8-240277 discloses a technique relating to a solenoid drive device mounted on a car, etc. The present invention incorporates the contents of these documents into the present specification by reference.
A conventional solenoid drive device has the following problems.
The control circuit, etc. of a solenoid drive device is usually contained in a control unit. However, the solenoid is set at a location apart from the control unit. Therefore, it is necessary to connect the control unit and the solenoid to each other with a plurality of leads. In a case where a plurality of solenoids are driven, and in a case where the distance between the solenoid and the control unit is long, etc., the amount of wire harnesses becomes large.
The control circuit detects an electric current flowing through the solenoid and uses it for feedback control for, for example, optimizing the electric current flowing through the solenoid and turning off the electric current. The direction and value of a solenoid current abruptly changes. Because of this, the control circuit performs its processes by detecting an electric current flowing through the solenoid and using an output signal obtained by averaging the detected value. However, even if an electric current flowing through the solenoid abruptly changes, an output signal of a smoothing circuit does not follow the change and thus is delayed. Therefore, when an output signal of the smoothing circuit is used for control of the driving of the solenoid, there is a fear that a high-speed control can not be obtained.
Further, in order to detect the value of an electric current flowing through the solenoid, generally, a resistor having a small resistance value is connected to the solenoid, and a voltage drop at this resistor is detected. Because an electric current from a power source and a regenerative current flow through this resistor, electricity loss is large, and the amount of heat generated is large. Because of this, this resistor can not be built on a monolithic IC (Integrated Circuit), which hinders reduction of the number of components and miniaturization of the solenoid drive device.